Java17的新特性 java17jdk17

Java语言特性系列

Java5的新特性
Java6的新特性
Java7的新特性
Java8的新特性
Java9的新特性
Java10的新特性
Java11的新特性
Java12的新特性
Java13的新特性
Java14的新特性
Java15的新特性
Java16的新特性
Java17的新特性
Java18的新特性

序本文主要讲述一下Java17的新特性
版本号

java -version openjdk version "17" 2021-09-14 OpenJDK Runtime Environment (build 17+35-2724) OpenJDK 64-Bit Server VM (build 17+35-2724, mixed mode, sharing)

从version信息可以看出是build 17+35

特性列表 JEP 306: Restore Always-Strict Floating-Point Semantics

恢复始终执行严格模式的浮点定义，修复25年前英特尔的浮点指令存在的一些问题

JEP 356: Enhanced Pseudo-Random Number Generators

引入RandomGenerator及RandomGeneratorFactory提供更好的随机数生成

RandomGenerator generator = RandomGeneratorFactory.all() .filter(RandomGeneratorFactory::isJumpable) .filter(factory -> factory.stateBits() > 128) .findAny() .map(RandomGeneratorFactory::create) //if you need a `JumpableGenerator`: //.map(JumpableGenerator.class::cast) .orElseThrow();

JEP 382: New macOS Rendering Pipeline

使用Apple Metal API为macOS新增了Java 2D internal rendering pipeline

JEP 391: macOS/AArch64 Port

迁移JDK到macOS/AArch64

JEP 398: Deprecate the Applet API for Removal

标记Applet API为废弃方便后续移除，具体如下

java.applet.Applet java.applet.AppletStub java.applet.AppletContext java.applet.AudioClip javax.swing.JApplet java.beans.AppletInitializer

JEP 403: Strongly Encapsulate JDK Internals

对JDK内部的api进行更强的封装，是 JEP 396: Strongly Encapsulate JDK Internals by Default的后续

JEP 406: Pattern Matching for switch (Preview)

引入switch模式匹配的preview版本，instanceof的模式匹配在JDK14作为preview，在JDK15作为第二轮的preview，在JDK16转正

static String formatterPatternSwitch(Object o) { return switch (o) { case Integer i -> String.format("int %d", i); case Long l-> String.format("long %d", l); case Double d-> String.format("double %f", d); case String s-> String.format("String %s", s); default-> o.toString(); }; }

JEP 407: Remove RMI Activation

移除Remote Method Invocation (RMI)，它在JDK15的 JEP 385被废弃

JEP 409: Sealed Classes

Sealed Classes在JDK15作为preview引入，在JDK16作为第二轮preview，在JDK17转正

package com.example.geometry; public abstract sealed class Shape permits Circle, Rectangle, Square, WeirdShape { ... }public final class Circle extends Shape { ... }public sealed class Rectangle extends Shape permits TransparentRectangle, FilledRectangle { ... } public final class TransparentRectangle extends Rectangle { ... } public final class FilledRectangle extends Rectangle { ... }public final class Square extends Shape { ... }public non-sealed class WeirdShape extends Shape { ... }

JEP 410: Remove the Experimental AOT and JIT Compiler

移除实验性的java版本的AOT及JIT Compiler，具体移除

jdk.aot — the jaotc tool jdk.internal.vm.compiler — the Graal compiler jdk.internal.vm.compiler.management — Graal's MBean

后续要使用可以使用GraalVM
JEP 411: Deprecate the Security Manager for Removal

废弃java1.0引入的Security Manager方便后续移除

JEP 412: Foreign Function & Memory API (Incubator)

JDK14的 JEP 370: Foreign-Memory Access API (Incubator)引入了Foreign-Memory Access API作为incubator，JDK15的 JEP 383: Foreign-Memory Access API (Second Incubator)Foreign-Memory Access API作为第二轮incubator，JDK16的 JEP 393: Foreign-Memory Access API (Third Incubator)作为第三轮，它引入了Foreign Linker API，JDK17引入Foreign Function & Memory API

JEP 414: Vector API (Second Incubator)

JDK16引入了 JEP 338: Vector API (Incubator)提供了jdk.incubator.vector来用于矢量计算，JDK17进行改进并作为第二轮的incubator

JEP 415: Context-Specific Deserialization Filters

允许应用去配置指定上下文及动态选择的deserialization filters，示例

public class FilterInThread implements BinaryOperator {// ThreadLocal to hold the serial filter to be applied private final ThreadLocal filterThreadLocal = new ThreadLocal<>(); // Construct a FilterInThread deserialization filter factory. public FilterInThread() {}/** * The filter factory, which is invoked every time a new ObjectInputStream * is created.If a per-stream filter is already set then it returns a * filter that combines the results of invoking each filter. * * @param curr the current filter on the stream * @param next a per stream filter * @return the selected filter */ public ObjectInputFilter apply(ObjectInputFilter curr, ObjectInputFilter next) { if (curr == null) { // Called from the OIS constructor or perhaps OIS.setObjectInputFilter with no current filter var filter = filterThreadLocal.get(); if (filter != null) { // Prepend a filter to assert that all classes have been Allowed or Rejected filter = ObjectInputFilter.Config.rejectUndecidedClass(filter); } if (next != null) { // Prepend the next filter to the thread filter, if any // Initially this is the static JVM-wide filter passed from the OIS constructor // Append the filter to reject all UNDECIDED results filter = ObjectInputFilter.Config.merge(next, filter); filter = ObjectInputFilter.Config.rejectUndecidedClass(filter); } return filter; } else { // Called from OIS.setObjectInputFilter with a current filter and a stream-specific filter. // The curr filter already incorporates the thread filter and static JVM-wide filter // and rejection of undecided classes // If there is a stream-specific filter prepend it and a filter to recheck for undecided if (next != null) { next = ObjectInputFilter.Config.merge(next, curr); next = ObjectInputFilter.Config.rejectUndecidedClass(next); return next; } return curr; } }/** * Apply the filter and invoke the runnable. * * @param filter the serial filter to apply to every deserialization in the thread * @param runnable a Runnable to invoke */ public void doWithSerialFilter(ObjectInputFilter filter, Runnable runnable) { var prevFilter = filterThreadLocal.get(); try { filterThreadLocal.set(filter); runnable.run(); } finally { filterThreadLocal.set(prevFilter); } } }// Create a FilterInThread filter factory and set var filterInThread = new FilterInThread(); ObjectInputFilter.Config.setSerialFilterFactory(filterInThread); // Create a filter to allow example.* classes and reject all others var filter = ObjectInputFilter.Config.createFilter("example.*; java.base/*; !*"); filterInThread.doWithSerialFilter(filter, () -> { byte[] bytes = ...; var o = deserializeObject(bytes); });

细项解读上面列出的是大方面的特性，除此之外还有一些api的更新及废弃，主要见JDK 17 Release Notes，这里举几个例子。
添加项

DatagramSocket Can Be Used Directly to Join Multicast Groups (JDK-8237352)

更新了java.net.DatagramSocket用于支持joining multicast groups
Console Charset API (JDK-8264208)

java.io.Console新增了方法用于返回console的charset
JDK Flight Recorder Event for Deserialization (JDK-8261160)

JDK Flight Recorder新增了jfr.Deserialization实现
Unified Logging Supports Asynchronous Log Flushing (JDK-8229517)

引入了 -Xlog:async参数用于异步日志，以及 -XX:AsyncLogBufferSize=用于控制buffer的大小

移除项

Removal of sun.misc.Unsafe::defineAnonymousClass (JDK-8243287)

移除sun.misc.Unsafe::defineAnonymousClass方法

废弃项

Deprecate 3DES and RC4 in Kerberos (JDK-8139348)

废弃了Kerberos的des3-hmac-sha1及rc4-hmac这两个encryption types
Deprecate the Socket Implementation Factory Mechanism (JDK-8235139)

废弃了如下几个工厂
static void ServerSocket.setSocketFactory?(SocketImplFactory fac) static void Socket.setSocketImplFactory?(SocketImplFactory fac) static void DatagramSocket.setDatagramSocketImplFactory?(DatagramSocketImplFactory fac)

已知问题

TreeMap.computeIfAbsent Mishandles Existing Entries Whose Values Are null (JDK-8259622)

TreeMap.computeIfAbsent方法针对null的处理与规范有偏差
Segmentation Fault Error on 9th and 10th Generation Intel? Core? Processors (JDK-8263710)

当运行在9th and 10th Generation Intel? Core? Processors时会有Segmentation Fault Error

其他事项

Updated List of Capabilities Provided by JDK RPMs (JDK-8263575)

xml-commons-api, jaxp_parser_impl, 以及java-fonts已经从OracleJDK/OracleJRE RPMs移除
New Implementation of java.nio.channels.Selector on Microsoft Windows (JDK-8266369)

对windows的java.nio.channels.Selector API采用了更具可扩展的方式实现，原来的实现没有被移除，可以使用 -Djava.nio.channels.spi.SelectorProvider=sun.nio.ch.WindowsSelectorProvider来继续使用
Parallel GC Enables Adaptive Parallel Reference Processing by Default (JDK-8204686)

针对Parallel GC默认开启了 -XX:ParallelRefProcEnabled
URLClassLoader No Longer Throws Undocumented IllegalArgumentException From getResources and findResources (JDK-8262277)

URLClassLoader的getResources、findResources不再抛出文档未定义的IllegalArgumentException